Manchester Encoding

Theory and UseBy Professor Tom Mavroidis

January 31,2001

What is Manchester Encoding ?

• It is a process by which a Synchronous clock encoding technique is used by the OSI physical Layer to encode the clock and data of a synchronous bit stream.

What is the difference?

• In this technique the binary data to be transmitted over the cable are not sent as a sequence of logical 1 & 0’s known as non return to zero (NRZ), instead the bit are translated into a different format that has a number of advantages over using straight binary.

Where is it used?

• A popular technique used for encoding data on magnetic disks.

• Also known as phase encoding• Problems include poor bit storage

density

Resides in the Physical Layer

• Physical Layer serializes the frame (converts it to a series of bits) and send it across a circuit to the destination.

Three types of encoding discussed

• NRZ - Non Return to Zero• RZ - Return to zero• Manchester encoding

NRZ , Non Return to Zero transmission

• Each data bit is represented by a level. High = logic 1 low = logic 0

Problem with NRZ

• Long runs of consecutive bits with the same value make it impossible to detect bit boundaries by the inability of the receiver to detect the clocking speed.

RZ - Return to zero

• Pulses used to represent bits• logic 1 = pulse logic 0 absence of

pulse

RZ - Advantages

• Clock signals can be extracted from the logic 1 signals.

• Halves the width of the pulse• Polar RZ sends pulses for both

logic 0 and logic 1• Requires an extra voltage level

Manchester Encoding

• Logic 1 is represented by a transition in a particular direction in the center of each bit.

• Opposite direction used to represent logic 0

Representation

• Logic 0 1 to 0 (downward transition at bit center)

• Logic 1 0 to 1 (upward transition at bit center)

Manchester Encoding

• Was named after its birthplace in Manchester, England

• Bit transitions do not always occur at the ‘bit boundaries.

• There is always a transition at the center of the bit

Improved clocking

• Allows the receiver to extract the clock signal

• Correctly decodes the value and timing of each bit

Receiver

• Reassembles the series of bits to form a frame and forwards the frame for processing by the link layer

• Frame is then CRC checked and a retransmission is requested if the checksum is invalid.

Representation of phase shift

Uses

• Manchester encoding is used to :• Transmit data across systems• Store data on magnetic media,

disks etc.• Communicate with Satellites

Conclusion

• Based on application different types of encoding can be used

• Manchester encoding exhibits poor bit storage density but improved reliability.

Links

• http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/man.html

• http://home.europa.com/~celiac/manch_enc.html• http://csshaun.cs.ru/ac/za/netnotes/netnotes003.html• http://www.pdp8.net/tu56/tu56.shtml